In the period reviewed, important steps were taken towards better understanding of sensory communication involving MHC genes: (a) With the Mating Preference assay, perception of MHC types was validated genetically by showing MHC mating bias among H-2 typed F2 segregants of crosses between MHC congenic mice. (B) A much simplified assay, with a differentially scented Y maze in which mice, trained by water deprivation and reward (reinforcement), distinguish MHC types, abolishes the complications of mating and vastly extends the prospects for detailed study of MHC sensory recognition. (c) Urines are as effective as mice as sources of MHC odors in the Y maze, which still further amplifies the opportunities for analysis. (d) MHC perception was confirmed in nearly 10,000 trials in the Y maze, with inbred and F2 segregant mice and their urines as sources of odor. Thus we are now in a highly favorable position to study, in the Y maze scented by mice or urines: (a) Recognition of the reinforced MHC type, when presented on different or variable genotypic backgrounds, as an index of the prominence of MHC sensory identity in relation to whatever total non-MHC sensory identity there may be. (b) Distinction of mice differing at sub-regions of the MHC and vicinity, as an index of genetic complexity of MHC recognition and to define least-complex systems for chemical study. (c) Distinction of H-2 mutant from non-mutant mice, a provocative probe for H-2 pleiotropism. (d) The chemistry of MHC- distinctive odors, probably multiple. We have tentative evidence that the blocking of pregnancy by strange males (Bruce Effect), which is subject to genetic variation in male potency and female susceptibility manifest in particular male-female genotypic combinations, is influenced by MHC types. We shall investigate whether genetic variation affecting induction of this neuro-endocrine response depends on concurrent sensory recognition of not only male pheromone but also the male's MHC type. If so, then another potential role for MHC perception in nature will be established.